Shift capacity as optional in custom shifts

Currently shift capacity is required when you set up your volunteer need with custom shifts. Many agency managers have reported frustration with this, because in reality they're getting volunteer referrals from avenues other than our site. Even if it were an option (which it's not, without having to delete and rebuild the shift entirely) It's creating additional work to expect agencies to constantly edit this number as they go.

There's also a few issues with an agency manager setting their shift capacity too low or too high because they have to set something. Too low: groups may pass over the opportunity thinking their help isn't needed. Too high: the numbers look far fetched (ex. the site says we still need 147 volunteers for a small volunteer committee meeting because the agency manager just threw out a # of 50 per meeting time).